1. Field of the Invention
This invention relates to organosiloxane compositions that cure by a platinum-catalyzed hydrosilylation reaction. More particularly, this invention relates to one-part organosiloxane compositions of this type that can be stored for extended periods of time at a temperature of about 25.degree. C. yet cure rapidly at temperatures above about 80.degree. C.
2. Description of the Prior Art
Organosiloxane compositions that cure by a hydrosilylation reaction between alkenyl radicals and silicon-bonded hydrogen atoms are well known. Hydrosilylation reactions are typically catalyzed by metals from the platinum group of the periodic table of the elements or compounds of these metals. A desirable feature of these compositions is their rapid cure rate, particularly at temperatures above about 50.degree. C. Curing of the composition begins even at temperatures of 25.degree. C. or below once the organosilicon compound containing the alkenyl hydrocarbon radicals is combined with the organohydrogensiloxane in the presence of the catalyst. Compositions of this type therefore cannot generally be stored for extended periods of time prior to use.
The chemical and patent literature disclose a variety of compounds and reaction products that will retard or inhibit curing of compositions by platinum-catalyzed hydrosilylation reactions for varying periods of time under ambient conditions. These catalyst inhibitors fall into two general classes.
One class of inhibitors is composed of materials that effectively inhibit a hydrosilylation reaction over a relatively wide temperature range and can be volatilized out of the composition when it is desired to cure the composition. Examples of this class of inhibitor include but are not limited to pyridine (U.S. Pat. No. 3,188,299), acrylonitrile (U.S. Pat. No. 3,344,111), acetylenic compounds (U.S. Pat. No. 3,445,420) and perchloroethylene (U.S. Pat. No. 3,383,356).
The second class of inhibitors are non-volatile at temperatures conventionally used to cure the compositions. The inhibitory effect of these materials is overcome by heating to the desired curing temperature. Examples of this type of inhibitor include reaction products of an organohydrogensiloxane, the platinum-containing hydrosilylation catalyst and an acetylenic alcohol (U.S. Pat. Nos. 3,989,666 and 4,336,364), organic phosphines and phosphites (U.S. Pat. No. 3,188,300), benzotriazole (U.S. Pat. No. 3,192,181), dialkyl carboxylic esters (U.S. Pat. Nos. 4,256,870 and 4,347,346) and unsaturated amides (U.S. Pat. No. 4,337,332).
The acetylenic compounds disclosed in the aforementioned U.S. Pat. No. 3,445,420 preferably boil at temperatures between about 25.degree. and 250.degree. C. under atmospheric pressure. This class of compounds includes hydrocarbons, alcohols, ketones and heterocyclic compounds containing at least one carbon-to-carbon triple bond per molecule. Preferred inhibitors are secondary or tertiary acetylenic alcohols and are present at a concentration of at least 0.1 weight percent, based on the weight of the curable composition.
A disadvantage of many organosiloxane materials prepared by curing compositions using a platinum-catalyzed hydrosilylation reaction is the relatively poor adhesion of these materials to inorganic substrates, particularly metals and organic polymers. As in the case of hydrosilylation catalyst inhibitors, the prior art describes a wide variety of compounds that can be incorporated as adhesion promoters into the curable organosiloxane composition.
For example, U.S. Pat. No. 4,460,739, which issued to Ashby on July 17, 1984, discloses using maleamidyl substituted trialkoxy- or triacyloxysilanes for this purpose.
The use of partial hydrolysis products of aliphatically unsaturated alkoxysilanes, such as vinyltrialkoxysilanes, as adhesion promoters for compositions comprising a vinyl terminated polyorganosiloxane, an organohydrogensiloxane crosslinking agent and a platinum catalyst is taught in U.S. Pat. No. 4,311,739, which issued to Hardman and Dujak on Jan. 19, 1982.
A number of patents teach imparting adhesion to polyorganosiloxane compositions that are curable to silicone elastomers using a platinum-catalyzed hydrosilylation reaction by incorporating organosilicon compounds containing various silicon-bonded reactive groups such as ester, epoxy or trialkoxysilylalkyl group into the ethylenically unsaturated polyorganosiloxane reactant. U.S. Pat. No. 4,245,079, which issued to Matsumoto and Murai on Jan. 13, 1981 teaches that when the reactive group is trialkoxysilylalkyl, the ethylenically unsaturated polyorganosiloxane reactant exhibits repeating units of the general formula --OSi(CH.sub.3)[RSi(OR').sub.3 ], where R is alkylene and R' is alkyl.
U.S. Pat. No. 4,033,924, which issued on July 5, 1977 and U.S. Pat. No. 4,082,726, which issued on Apr. 4, 1978 disclose adhesion-promoting additives that are organosilicon compounds containing an epoxy group and at least one low molecular weight alkenyl radical or silicon-bonded hydrogen atom.
Organosilicon compounds containing at least two silicon bonded hydrogen atoms per molecule and an average of at least one unit of the formula (RO).sub.3 Si(CH2).sub.x (CH.sub.3)SiO and/or (RO).sub.3 Si(CH.sub.2).sub.x (CH.sub.3).sub.2 SiO.sub.1/2 where R represents methyl, ethyl or CH.sub.3 C(O)-- and x is 2 or 3 are taught in U.S. Pat. No. 3,772,066, which issued to Clark and Hays on Nov. 13, 1973. In addition to functioning as curing agents these organosilicon compounds also improve the adhesion of the cured compositions.
Incorporation of epoxy groups together with alkenyl radicals into the organosilicon compound that reacts with silicon bonded hydrogen atoms to form a cured product is taught in U.S. Pat. No. 4,157,357, which issued to Mine and Yokoyama on June 5, 1979.
The prior art also teaches using mixtures of two or more organosilicon compounds to achieve cohesive bonding between silicone elastomers cured by a hydrosilylation reaction and various substrates while extending the working time of the corresponding curable compositions. The combination of (1) a polyorganosiloxane containing a hydroxy group and a vinyl radical and (2) an epoxy-functional silane is taught in U.S. Pat. No. 4,087,585 that issued to Schulz on May 2, 1978. This patent also discloses the necessity of heating curable compositions containing this adhesion promoter to temperatures of at least 100.degree. C. to overcome the inhibiting effect of the adhesion promoter on the platinum-catalyzed curing reaction.
U.S. Pat. No. 4,810,763, which issued on July 3, 1989 teaches adding a carboxylic acid to extend the working time of organosiloxane compositions containing an amino-substituted organosilicon compound as an adhesion promoter.